1. Field of the Invention
This invention relates to an apparatus for extracting a specific signal from a television signal source, multiplexing it with a television signal, transmitting and receiving the multiplexed signal, and extracting the specific signal from the multiplexed signal.
2. Description of the Prior Art.
More than 35 years have passed since the color television broadcasting of the current NTSC (National Television System Committee) system began in 1954. Recently, in search of finer definition and higher performance television systems, several new systems including HDTV (High Definition Television) systems have been proposed. At the same time, the contents of the programs presented to viewers have been changed from the mere studio programs to programs providing higher quality images and more realistic feeling such as cinema-size movies.
The current NTSC broadcasting is standardized by 2:1 interlaced 525 scanning lines, a luminance signal bandwidth of 4.2 MHz, and an aspect ratio of 4:3. (See, for example, Pritchard, "US Color Television Fundamentals-A Review", IEEE Trans. Consumer Electron., vol. CE-23, pp. 467-478, November 1977).
In this background, several television signal composition methods aiming at compatibility with the current broadcasting system and enhancement of horizontal resolution have been proposed. One of such examples is presented in a paper of Faroudja and Roisen, "Improving NTSC to achieve near-RBG performance", SMPTE J., vol. 96, pp. 750-761, August 1987. They use a comb filter to split luminance and chrominance signals at the transmitting end and avoid crosstalk between them at a receiver. This method is useful for eliminating an annoying crosstalk on the received image, but horizontal and vertical high frequency components of the luminance signal cannot be transmitted, nor can the enhancement of the resolution be attained. Another example is presented in a paper of Fukinuki and Hirano, "Extended Definition TV Fully Compatible with existing Standards", IEEE Trans. Commun., vol. COM-32, pp. 948-953, August 1984. Considering the NTSC television signal expressed on a two-dimensional plane of temporal frequency f1 and vertical frequency f2, the chrominance signals C are present in the second and fourth quadrants due to their phase relationships to the chrominance subcarrier fsc. The Fukinuki et al example uses the vacant first and third quadrants for multiplexing the high frequency components of the luminance signal. These vacant quadrants are called the "Fukinuki Hole" after the inventor. The chrominance signal and the multiplex high frequency components are separated and reproduced at the receiving end, thereby enhancing the horizontal resolution. In this example, the conventional NTSC receiver would be interfered with by the multiplex signal, because the example has no ability for separating the chrominance signal from the multiplex high frequency components. In the current television broadcast, as is clear from the description above, the signal bandwidth is limited by the standard, and it is not easy to add some new information with a high quality. For example, other methods to enhance the horizontal resolution have been proposed (M. Isnardi et al, "A Single Channel NTSC Compatible Widescreen EDTV System", HDTV Colloquium in Ottawa, October, 1987), but many problems are left unsolved from the viewpoint of the compatibility with the current television broadcasting and the deterioration of demodulation characteristics of the high frequency components in a moving picture. Besides, from the standpoint of effective use of the frequency resources, the transmission band cannot be easily extended.
The present inventors invented a method of superposing a signal by using quadrature modulation of the video carrier (U.S. Pat. No. 4,882,614 which issued Nov. 21, 1989, or see Yasumoto et al, "An extended definition television system using quadrature modulation of the video carrier with inverse Nyquist filter", IEEE Trans. Consumer Electron., vol. CE-33, pp. 173-180, August 1987). By this method, various signals can be transmitted using the newly established quadrature channel and the interference to the conventional NTSC receiver is very small in principle. But the interference can be detected in practice, because of the imperfectness of the characteristics of filters at the receiver and transmitter.
This invention is one solution to avoid imperfectness of those systems mentioned above. Even if the imperfectness of such filters/circuits occur, the interference to the conventional NTSC receivers can be reduced down to an acceptable level. In this sense, this invention is very useful when one transmits the multiplex signal using quadrature modulation of the video carrier.